1. Field of the Invention
The present invention relates to an improved photoresist material, and, more particularly, to a photoresist material having an improved layer of a photopolymerizable composition on a support.
2. Description of the Prior Art
Hitherto, solution type photoresist materials such as a photosensitive liquid comprising a combination of a dichromate and glue or a dichromate and polyvinyl alcohol, a photosensitive liquid mainly comprising polyvinyl cinnamate and sensitizers, and a photosensitive liquid mainly comprising a natural rubber or a cyclized rubber and a crosslinking agent have been largely used as photoresist materials for making printed circuits.
Furthermore, as another type of photoresist material, there is a film type photoresist material comprising two plastic films and a photosensitive or photopolymerizable layer sandwiched between the plastic films. Upon use of this type of photoresist material, one of the plastic films is separated to expose the photosensitive layer, the photosensitive layer having another plastic film at the back side thereof is superposed on a copper base plate for making printed circuits, the assembly is image-wise exposed from the plastic film bearing side, the plastic film is then stripped, and the exposed layer is developed in a suitable liquid developer such as an organic solvent or an aqueous alkaline solution. The unexposed portions of the photosensitive layer are dissolved away by the developer while the exposed portions (which were hardened by the action of the actinic radiation) are left on the copper base plate. Thus, a resist pattern for printed circuits is formed on the copper plate.
However, in the above mentioned solution type or film type photoresist materials, there is the troublesome problem that the development requires a liquid developer such as an organic solvent or an aqueous alkaline solution, as well as such a technique is undesirable from the view point of environmental pollution since the use of such a liquid developer may cause pollution problems. Dry developable recording materials have recently been developed in place of such solution development type recording materials, e.g., as are described in Japanese Patent Publication No. 9663/'63 and U.S. Pat. Nos. 3,353,955 and 3,770,438, resist images or patterns are formed by utilizing the difference in adhesive strength to a support between exposed portions and unexposed portions of a layer of photosensitive or photopolymerizable composition. According to this method, the layer of a photopolymerizable composition mainly comprising a polymer binder, an unsaturated monomer, and a photopolymerization initiator is formed on a base support such as a plastic film, a metal plate, a paper, etc., a transparent film is superposed on the layer as a cover, the photosensitive layer is exposed through the cover film and through an original, and then the cover is separated to leave the exposed portions or unexposed portions of the photosensitive layer on the base support while leaving the other portions on the cover film. Thus, a negative and positive (or a positive and negative) of the original image are formed on the base support and the cover film, respectively. In the case of using this type of recording material, developing by a liquid developer as is required in the case of using the aforementioned solution development type recording materials is unnecessary and resist patterns are formed by stripping the cover film. This method is called "stripping development" and the photoresist material used in the method is called a "stripping development type photoresist material". The use of this type of photoresist material in making printed circuits provides large advantages.
Still further, in the case of employing stripping development type photoresist materials, the resist can be quite easily removed as compared with the case of employing the conventional solution development type photoresist materials, and, thus, the stripping development method is said to be a very advanced technique which is easy to use on a commercial scale.
In the case of making resist patterns using stripping development type photoresist materials, it is necessary that the photosensitive layer be broken or split after exposure at the interface between exposed portions and unexposed portions thereof. For this purpose, the properties of the polymer binders and the monomers used in such photoresist materials are particularly important. Among these properties, the properties of the polymer binders, such as, for example, the molecular weight, the softening point, the crystallinity, the miscibility with other elements, and the adhesivity to supports are important points. Furthermore, the images or patterns of the resist thus formed are used for various purposes and hence it is preferred that the resist images or patterns possess properties meeting the end use environment, for example, possess high abrasion resistance, weatherability, etc.
As mentioned above, in a photopolymerizable or photosensitive material comprising a combination of a polymer as a binder, a monomer, and a polymerization initiator, the properties of the elements used in the material are important, but few of the conventional elements used in the photopolymerizable materials have satisfactory properties and they do not give sufficient images or patterns. For example, in the technique disclosed in Japanese Patent Publication No. 9663/'63 polymers are used as a binder and tackifier. As such polymers, polyvinyl butyral, polyvinyl acetate, polyvinyl pyrrolidone, gelatin, an indene-coumarone resin, a silicone resin, rubber, etc., are used. Also, in U.S. Pat. No. 3,770,438 it is described that a vinylidene chloride copolymer, cellulose ether, a synthetic rubber, a polyvinyl acetate copolymer, polyacrylate, polyvinyl chloride, etc., are used as "binder". However, the use of a combination of these polymers, various monomers, and the polymerization initiators for the monomers does not always give sufficient results upon practical use.
As a result of various investigations to overcome these difficulties in conventional techniques, the inventors previously found that chlorinated polyolefins possessed excellent properties as polymers for the binder, and, based on this discovery, made an invention related to a photosensitive material comprising a support having formed thereon a layer of a photopolymerizable composition, and, if necessary, a protective layer formed on the photosensitive layer, the polymerizable composition comprising a chlorinated polyolefin, a monomer having an addition polymerizable unsaturated bond, and a photopolymerizable initiator as described in Japanese Patent Application (Laid Open) No. 123,012/'74 (German Patent Application Offenlegungsschrift No. 2,414,240). However, although the layer of the photopolymerizable composition of the aforesaid invention could provide good resist patterns formed on the surface of a base material by laminating the layer on the surface of the base material, such as a copper plate and an aluminum plate, image-wise exposing the layer, and then stripping the support, it was also found that the exposed or unexposed portions of the layer which had to be left on the base material were partially removed therefrom by the support at stripping to cause cutting or partial defects of the resist patterns on the surface of the base material. This phenomenon is believed to be caused by the fact that the layer of the photopolymerizable composition does not have sufficient and uniformly strong adhesivity to the base material.